This invention relates to a focus detecting device in a single lens reflex camera. In particular, it relates to an SLR focus device which electrically detects proper focus by utilizing the contrast of an object's image, or the variation in light and shade thereof when it becomes a maximum value.
There have been proposed in the prior art a number of electrical focus detecting methods. For instance, an electrical focus detecting method utilizing the variations of spatial frequencies of objects is disclosed in Published Unexamined Japanese Patent Application No. 56934/1975. An electrical focus detecting method utilizing the contrast of an image is shown in U.S. Pat. No. 3,688,673, and an electrical focus detecting method in U.S. Pat. No. 4,002,899 utilizes the fact that the image of an object is processed through two optical paths to obtain two images of the object. The two images are made to coincide with each other when the focalization is obtained.
However, the first method is not suitable for a single lens reflex camera because it is difficult to eliminate the movable parts of the electrical focus detecting device. Therefore, the device itself tends to be bulky and requires a large power source to drive the movable parts.
The second method is also disadvantageous in that it is necessary to provide a number of elements for detecting focus, namely, photo-electric conversion elements, and therefore the detecting circuit is rather intricate.
The third method requires a large number of movable parts as in the first method. In addition, the detection accuracy depends on the accuracy in relative position of the elements for detecting two images of the object. Therefore it is necessary to align the positions of the elements with a high degree accuracy. Furthermore, it is very difficult to form two optically equal images of an object with two optical paths in a single lens reflex camera.
Thus, problems to be solved exist if the conventional electrical focus detecting methods are to be used in a practical system in a single lens reflex camera.